Method of making a decorative alloy



Patented Aug. 4, 1936 UNITED STATES PATENT OFFICE METHOD OF MAKING ADECORATIVE ALLOY No Drawing. Application March 12, 1932, Serial No.598,535

3 Claims.

This invention has for its object the production of an alloy with amicrostructure consisting of very fine approximately parallel lame-llae,which result in the diffraction of light and produce an iridescentpearl-like effect, rendering the alloy suitable for use in thedecorative and architectural arts.

It has long been known that if fine parallel lines are ruled on apolished surface of metal, the

' so-called diffraction grating resulting will produce a series ofspectra showing all colors present in the incident light diffracted atdifierent angles. The effect is customarily explained on the basis ofthe wave theory of light as being due to the interference of light wavesreflected by the successive parts of the surface, the angle ofdiffraction depending on both the spacing of the rulings and the wavelength of the light concerned, according to the well known formula wheren is a whole number indicating the order of the reflection, A is thewave length of the light, d the distance apart of the rulings in thegrating and sin and sin 5 the sines of the angles made by the incidentand diffracted rays respectively. To produce large angles of diffractionfor use in scientific spectroscopy, rulings of 10,000 to 25,000 linesper inch are customarily used, although the effect is apparent even withrulings 1/500 apart and disappears only when the spacing is equal to orless than the wave length of light employed. It is the purpose of thisinvention to produce a type of diffraction grating by the use of ametallic alloy of such composition that by suitable heat treatment awell developed fine plate structure results. Such an alloy showsdifferent colors at different angles and is very suitable for employmentin decorative metal Work in building construction and for the productionof small objects or parts wherein the intrinsic beauty of the metal isof advantage. Specifically, the structure is developed by heat treatmentof an alloy which consists of a uniform single micro-constituent orphase at high temperatures and which breaks raphers for some time.Nevertheless no attempt has been made to develop the structures to showiridescence or use it for decorative purposes because of its inherentbeauty.

While the striated structure under certain conditions produced incertain alloys due to a change of solubility of a second phase in theprinciple one produces the effect to some extent, the best results areobtained in the case of the so-called eutectoid alloys. The termeutectoid is that given by metallographers to any alloy in thetemperature-composition diagram of an alloy series which changes from ahomogeneous single solid phase above a certain temperature to twodifferent ones below this temperature. Heat treatment of such an alloyresults in the greatest quantity of precipitated material and, ifsuitably controlled, to the brightest reflection and greatest colorafter suitable polishing and etching. The structures are notcharacteristic of only one alloy, but result from a certain type oftransformation and occur in many alloy series. The invention is notlimited to any one system but refers to the development of suchstructures in any alloys which undergo the change in phase withtemperature, for use as decorative material. For example, successfulresults have been achieved using the alloys approximating in compositionthe eutectoids in the copper-aluminum, copper-tin, copper-siliconsystem, while the ternary eutectoid alloys, copper-zinc tin andcopper-aluminum-silicon, have also shown satisfactory results. Theaddition of a third or fourth element is often advantageous in modifyingthe structure. Copper is not an essential constituent and suitableresults can be obtained using any other system of two or more metals inwhich suitable microstructure can be obtained.

In its broadest aspects the method of developing the desired structurecomprises heating the alloy to a temperature which produces a singlehomogeneous phase which can be determined from the equilibrium diagramof the alloy system being considered, then with or without anintermediate cooling to room temperatures bringing the alloy to atemperature which results in the formation of two different phases, andthen polishing and etching as later described.

To give a specific example of the treatment required, the followingrefers to treatment which has been found satisfactory for thecopperaluminum alloy containing 11.9 per cent aluminum. The alloy isfirst cast, hot rolled, extruded or otherwise fabricated into thedesired form and is then heated to a temperature above the eutectoidtemperature, that is according to my investigation approximately 569 C.,but preferably to about 900 C., in order to develop large grains of thebeta phase stable above the eutectoid temperature. Grains of abnormallylarge size, up to 3 inches or even more in diameter, may be developed bycooling the alloy quickly to room temperature and reheating to about 900C. The alloy is then cooled to about 550 C. and held at this temperaturefor a sufficient length of time to enable the alloy to decomposecompletely and give rise to well defined lamellar structure. Or statedin another way the single homogeneous beta phase breaks down to form amixture of the alpha and delta phases in the form of fine approximatelyequidistant and parallel plates. Treatment at lower temperatures, butabove 400 C., will result in quicker transformation but will give astructure too fine for the best results. An alternative and thepreferred method is to cool the alloy rapidly, for example by quenching,fom 900 C. to room temperature or considerably below the eutectoidtemperature and then to reheat it to a temperature high enough to permitthe reaction to take place, but not above the eutectoid temperature, inthe above example in the neighborhood of 500 C., when the precipitatedplates will be straighter, and more evenly distributed than thoseresulting from treatment by the first method. Such a structure iscommonly called a Widmanstatten structure and does not result fromdirect transformation at 500 C.

There are several alloys which produce a suitable structure on directslow solidification from the liquid state and are therefore suitable foruse in the form of castings without further treatment. Such alloys areknown as eutectics and the use of these on account of their iridescentappearance is included in the present invention. The copper-phosphoruseutectic containing approximately 8.3 per cent. phosphorus has beenfound to give very satisfactory results, providing it is allowed tosolidify slowly.

It is also possible by the use of suitable etching to developinterference colors in alloys of fine duplex structure even if theconstituents are not arranged in the form of parallel plates, sinceinterference then occurs between light reflected from the high and lowparts of the etched surface.

The surface of alloys in which the structures have been developed inaccordance with the method described above need to be polished andetched in order to bring out the effect to its fullest extent. Etchingmay be accomplished in several ways but is best performed with a reagentwhich differentially chemically attacks the different phases of thestructure and leaves one in relief without producing a film or stainingon either. Satisfactory results have been obtained on thealuminum-copper alloys mentioned above by the use of solutions ofammonium hydroxide and hydrogen peroxide; ammonium persulphate withammonium hydroxide; and mixtures of chromic and nitric acids. However,any etching reagent customarily employed for the development of thestructure of the alloys may be employed. Every different compositionwill obviously require a special study of etching methods.

The etched surface produced as above described may be covered withlacquer or other transparent protective film in order to preventatmospheric tarnishing, with little detriment to its appearance.

Having thus set forth the nature of my invention, what I claim is:

1. A method of making an architectural or decorative shaped metal objectwhich comprises forming and shaping the object to the desired finishedform of an architectural or decorative 10 shape of a copper-aluminumalloy of approximately eutectoid composition, heating the formed andshaped object to a temperature above the eutectoid temperature todevelop large grains of the phase which is stable above the eutectoid 15temperature, cooling the object rapidly to a temperature considerablyunder the eutectoid, reheating it to and maintaining it at a temperaturenot exceeding the eutectoid temperature but sufficiently high to permitdecomposition of the 20 alloy into two different phases distributed inthe form of thin plates, then cooling, and then polishing and etchingthe surface of the object with a reagent which differentially chemicallyattacks the diiferent phases of the structure in such a manner as toproduce a surface which gives an iridescent effect due to thediffraction of light.

2. A method of making an architectural or decorative shaped metal objectwhich comprises forming and shaping the object to the desired 3 finishedform of an architectural or decorative shape of a copper-aluminum alloycontaining in the neighborhood of 11.9% aluminum, heating the object toa temperature above 569 C. to develop large grains of the phase which isstable above the eutectoid temperature, cooling the object and maintaingit at a temperature of about 550 C. for a sufficient time to permitdecomposition of the alloy into two different phases distributed in theform of lamellae, then cooling, and then polishing and etching thesurface of the object with a reagent which differentially chemicallyattacks the different phases of the structure in such a manner as toproduce a surface which gives an iridescent effect due to thediifraction of light.

3. A method of making an architectural or decorative shaped metal objectwhich comprises forming and shaping the object to the desired finishedform of an achitectural or decorative shape of a copper-aluminum alloycontaining in the neighborhood of 11.9% aluminum, heating the object toa temperature above 569 C. to develop large grains of the phase which isstable above the eutectoid temperature, cooling the object rapidly to atemperature considerably below the eutectoid temperature, reheating theobject to a temperature in the neighborhood of 500 C. and maintaining atthis temperature for a sufii- 6 cient time to permit decomposition ofthe alloy into two different phases distributed in the form of lamellae,then cooling, and then polishing and etching the surface of the objectwith a reagent which differentially chemically attacks the differ- 5 entphases of the structure in such a manner as to produce a surface whichgives an iridescent effect due to the diffraction of light.

CYRIL STANLEY SMITH.

